1. Field of the Invention
The present invention relates to a method for producing a silicon ingot having a directional solidification structure with a suitable degree of orientation for producing a silicon substrate for use in photovoltaic solar cells, especially to a method for producing a silicon ingot having a directional solidification structure with a wide horizontal cross-sectional area and good degree of orientation.
2. Description of the Related Art
Silicon substrates comprising polycrystalline silicon have been known in the art as a kind of silicon substrate for use in photovoltaic solar cells. The silicon substrate comprising the polycrystalline silicon is produced by slicing a silicon ingot having a directional solidification structure. Although the silicon ingot having a directional solidification structure is thought to be cheaper than the single crystal silicon, many researches are under way for satisfying requirements for much cheaper silicon substrates.
FIG. 5 denotes an illustrative cross section for describing the method for producing a conventional silicon ingot having a directional solidification structure. As shown in FIG. 5(a), the method comprises the steps of filling a raw silicon material 2 in a melting crucible 1, melting the raw silicon material 2 by heating the melting crucible 1 with an induction coil 3, and injecting the molten silicon 8 into a solidification crucible 4 as shown in FIG. 5(b). An insulating heater 5 is provided around the solidification crucible 4 and a baffle 6 for shielding the heat from the heater 5 is additionally provided at the bottom end of the insulating heater 5. A chill plate 7 makes contact with the bottom of the solidification crucible 4. The injected molten silicon in the solidification crucible 4 starts to solidify from the bottom to the top since the bottom of the solidification crucible 4 is chilled with the chill plate 7. An elevator shaft 11 is further provided at the bottom face of the chill plate 7, a directional solidification structure 12 being grown over the entire region of the molten silicon liquid by allowing the molten silicon 8 to solidify from the bottom while the chill plate 7 is descending using the elevator shaft 11 at a velocity synchronizing with the crystal growth speed of the molten silicon. The silicon substrate for use in the photovoltaic solar cell is produced by slicing the silicon ingot after shaving off the side wall part of the silicon ingot, since the side wall of the silicon ingot having a directional solidification structure obtained as described above contains a high concentration of impurities introduced from the crucible 4 as well as a lot of distortion fault.
However, when the silicon ingot obtained has a small horizontal cross-sectional area, the horizontal cross-sectional area of the silicon ingot after shaving off the side wall part becomes still smaller along with increasing the silicon ingot side wall elimination ratio, making it impossible to effectively utilize the expensive silicon raw material.
Accordingly, a silicon ingot having a directional solidification structure with a wide horizontal cross-sectional area as well as an excellent degree of orientation is desired. However, when one attempts to produce a silicon ingot having a wide horizontal cross-sectional area by the conventional production method in which the temperature is controlled by disposing heat generation sources on side faces, the degree of orientation along the vertical (i.e., solidification) direction becomes poor due to temperature differences caused along the horizontal direction, making it impossible to obtain a silicon ingot having a suitable degree of orientation.
Accordingly, the object of the present invention is to provide a silicon ingot having a good degree of orientation with a wider horizontal cross-sectional area than the conventional silicon ingot.
In one aspect, the present invention provides a method for producing a silicon ingot having a directional solidification structure comprising the steps of: placing a silicon raw material into a crucible of a melting device constructed by mounting a chill plate capable of chilling with a refrigerant on an underfloor heater, mounting the crucible on the chill plate, providing an overhead heater over the crucible, and surrounding the circumference of the crucible with a heat insulator; heat-melting the silicon raw material by flowing an electric current through the underfloor heater and overhead heater while halting refrigerant feed to the chill plate, followed by halting the electric current or decreasing the electric power through the floor heater after the silicon raw material has been completely melted, the molten silicon being chilled from the bottom of the crucible by chilling the chill plate by feeding the refrigerant; and intermittently or continuously lowering the temperature of the overhead heater by intermittently or continuously decreasing electric current through the overhead heater along with halting the electric current or decreasing the electric power through the floor heater.
In another aspect, the present invention provides an apparatus for producing a silicon ingot having a directional solidification structure, where the apparatus is equipped with an underfloor heater, a chill plate mounted on the floor heater, a crucible mounted on the chill plate, an overhead heater provided over the crucible and a heat insulator surrounding the circumference of the crucible.